/*
 * OREKIT-X
 * Copyright 2002-2008 CS Communication & Systemes
 * 
 * Parts of this software package have been licensed to CS
 * Communication & Systemes (CS) under one or more contributor license
 * agreements.  See the NOTICE file distributed with this work for
 * additional information.
 *  
 * This is an experimental copy of OREKIT from www.orekit.org.
 * Please use the original OREKIT from orekit.org for normal work
 * unrelated to this research project.
 * 
 * Licensed under the Apache License, Version 2.0 (the "License"); you
 * may not use this file except in compliance with the License.  You
 * may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
 * implied.  See the License for the specific language governing
 * permissions and limitations under the License.
 */
package ore.propagation.events;

import ore.Orbit;
import ore.PositionVelocity;
import ore.SpacecraftState;
import ore.errors.OrekitException;

import org.apache.commons.math.geometry.Vector3D;

/** 
 * Detector of apside crossing events.
 * 
 * This class identifies apside crossing events (i.e. apogee or
 * perigee crossing).
 * 
 * The default implementation behavior is to continue propagation at
 * apogee crossing and to stop propagation at perigee crossing.  This
 * can be changed by overriding the eventOccurred method in a derived
 * class.
 * 
 * Beware that apside detection will fail for almost circular
 * orbits. If for example an apside detector is used to trigger an
 * {@link ore.forces.maneuvers.ImpulseManeuver ImpulseManeuver} and
 * the maneuver change the orbit shape to circular, then the detector
 * may completely fail just after the maneuver has been performed!
 * 
 * @author Luc Maisonobe
 */
public class ApsideDetector
    extends AbstractDetector
{

    /** 
     * The orbit is used only to set an upper bound for the max check
     * interval to (period/3) and to set the convergence threshold
     * according to orbit size
     * 
     * @param orbit Initial orbit
     */
    public ApsideDetector(Orbit orbit) {
        super((orbit.getKeplerianPeriod() / 3),
              (1.0e-13 * Math.sqrt(orbit.getMu() * orbit.getA())));
    }


    /** 
     * Handle an apside crossing event and choose what to do next.
     * 
     * The default implementation behavior is to {@link
     * EventDetector#CONTINUE continue} propagation at apogee crossing
     * and to {@link EventDetector#STOP stop} propagation at perigee
     * crossing. This can be changed by overriding this method in a
     * derived class.
     * 
     * @param s The current state information : date, kinematics, attitude
     * @param increasing If true, the value of the switching function increases
     * when times increases around event (note that increase is measured with respect
     * to physical time, not with respect to propagation which may go backward in time)
     * 
     * @return One of {@link #STOP}, {@link #RESET_STATE}, {@link #RESET_DERIVATIVES}
     * or {@link #CONTINUE}
     * 
     * @exception OrekitException Some specific error occurs
     */
    public int eventOccurred(SpacecraftState s, boolean increasing)
        throws OrekitException
    {
        return increasing ? STOP : CONTINUE;
    }
    /**
     * {@inheritDoc}
     */
    public double g(final SpacecraftState s) throws OrekitException {
        PositionVelocity pv = s.getPositionVelocity();
        return Vector3D.dotProduct(pv.getPosition(), pv.getVelocity());
    }
}
